1. Field of the Invention
This invention relates to an electrophotographic endless belt, and also relates to a process cartridge and an electrophotographic apparatus which have an electrophotographic endless belt.
2. Related Background Art
Besides rigid-body, drum-shaped members, flexible endless-belt-shaped members (electrophotographic endless belts) are conventionally used in intermediate transfer members, electrophotographic photosensitive members, transfer-transport members, fixing members and so forth used in electrophotographic apparatus, such as copying machines and laser beam printers.
Usually, in an electrophotographic apparatus, an electrophotographic endless belt is stretched over, and supported on, at least two rollers (stretch-over rollers) disposed on its inner-periphery side and is rotatingly driven under the application of any desired tension when used.
However, because of frequently possible slight errors or scattering in the diameter, deflection, rotating-shaft straightness and roller-to-roller parallelism of the stretch-over rollers stretchedly supporting the electrophotographic endless belt, it is difficult to prevent the electrophotographic endless belt from meandering from side to side during its rotating drive.
Such meandering of the electrophotographic endless belt from side to side makes the exposure position and the transfer position deviate to cause image misregistration. Also, in the case of a full-color electrophotographic apparatus, it makes the position of image formation deviate for each color to cause color misregistration when color toner images are superimposed on the electrophotographic endless belt or on a transfer material transported on the electrophotographic endless belt.
Accordingly, in order to prevent the electrophotographic endless belt from meandering like this, various methods have ever been proposed. In these recent years, a large number of methods have been proposed in which a meandering-preventive member is provided on the inner peripheral surface of a beltlike substrate to prevent the electrophotographic endless belt from meandering.
For example, a method is available in which stretch-over rollers, each provided over the whole outer peripheries thereof with a groove that may fit in the cross-sectional shape of such a meandering-preventive member, are used and an electrophotographic endless belt provided with the meandering-preventive member over the whole inner peripheral surface is rotated, making the meandering-preventive member fit in each groove of the stretch-over rollers to prevent the electrophotographic endless belt from meandering.
As another example, a method is available in which stretch-over rollers each having substantially the same length as the distance between inner sides of meandering-preventive members provided on both ends of a beltlike substrate of an electrophotographic endless belt are used and the electrophotographic endless belt is stretched over these stretch-over rollers and is rotated making its both-end meandering-preventive members and the stretch-over rollers fit in each other to prevent the belt from meandering.
As still another example, a method is available in which stretch-over rollers, each provided on one end in the axial direction thereof with a terraced portion in which a meandering-preventive member of an electrophotographic endless belt fits, are used to prevent the electrophotographic endless belt from meandering.
The above methods can make the electrophotographic endless belt travel smoothly without bringing it into a meandering state. This enables the formation of good images free of any image misregistration or color misregistration.
As methods by which the beltlike substrate is provided with the meandering-preventive member, the following methods are known in the art.
Japanese Patent Application Laid-Open No. S57-214167 discloses a method in which the meandering-preventive member is bonded with an adhesive to the inner peripheral surface of the beltlike substrate. This method has a disadvantage that the handling of the adhesive itself is troublesome and is automatable with difficulty. There is also a problem that the attachment position may gradually change because of the flowability of the adhesive, unless the meandering-preventive member and the beltlike substrate are kept firmly held by some means until the adhesive hardens completely. This makes it difficult to improve productivity because the meandering-preventive member must be fastened and supported so as not to shift. This method also has disadvantages that the adhesive may slip to lower the attachment precision of the meandering-preventive member. There is a further problem that it is difficult to control the adhesive not to protrude and this may adversely affect the travel performance of the electrophotographic endless belt.
In such a method, the electrophotographic endless belt has a superior durability, but has a poor attachment precision, and is producible so inefficiently as to result in a high cost. Moreover, this method tends to harm the operating environment. Thus, this method has been unsatisfactory.
A method is also available in which, as disclosed in Japanese Patent Application Laid-Open No. S59-230590, a hotmelt pressure-sensitive adhesive is used as a meandering-preventive member as it is, and also a method is available in which, as disclosed in Japanese Utility Model Publication No. H07-45092, a cold-curing silicone rubber is coated so as to be used as a meandering-preventive member as it is. In such methods, however, it is difficult to produce meandering-preventive members having a constant accuracy in shape and dimension, and the meandering-preventive member obtained has such a poor straightness as to put obstacles in the way of stable travel of the electrophotographic endless belt. It also lacks in productivity and automation adaptability, and involves a high cost. Thus, this method can not be said to be a preferable method.
To solve the above problems and the cost of as electrophotographic apparatus have become lower in recent years, a method is proposed in which, for the purpose of attaching the meandering-preventive member to the beltlike substrate at a low cost and with good precision, the meandering-preventive member is attached (stuck) to the beltlike substrate not by the method described above, but by the use of a pressure-sensitive adhesive, double-coated tape. The method making use of a pressure-sensitive adhesive, double-coated tape has the advantages that it is easy to handle because of its bond strength immediately after sticking, permitting it to be positioned with ease, and it is suited for automation. As a result, the meandering-preventive member can be stuck to the beltlike substrate with a good dimensional accuracy.
The method in which the meandering-preventive member is attached (stuck) to the beltlike substrate by the use of a pressure-sensitive adhesive, double-coated tape has a great advantage as described above. It, however, is not the case that the meandering-preventive member can be attached with a good precision whatever pressure-sensitive adhesive, double-coated tape is used.
For example, where as disclosed in Japanese Patent Application Laid-Open No. S62-50873, a pressure-sensitive adhesive, double-coated tape having no “reinforcing base material,” and consisting of only a pressure-sensitive adhesive, is used when the meandering-preventive member is attached to the beltlike substrate, the pressure-sensitive adhesive, double-coated tape may expand and contract in accordance with the expansion and contraction of the meandering-preventive member, so that the meandering-preventive member may be stuck to the beltlike substrate in the state in which the former has partially expanded or contracted. Thus, it is difficult to attach the meandering-preventive member to the beltlike substrate with good precision.
The pressure-sensitive adhesive, double-coated tape consisting of only a pressure-sensitive adhesive also has a problem that it has so low a peel strength as to slip gradually with the lapse of time when used in an environment of high temperature. Thus, this tape has a disadvantage that it can not endure long term use.
As a method which can solve the above problems, Japanese Patent Application Laid-Open No. H07-187435, e.g., discloses an example in which a pressure-sensitive adhesive, double-coated tape having a reinforcing base material is used.
The use of a reinforcing base material enables improvement in attachment precision. However, its bond strength and its maintenance is inadequate.
As a pressure-sensitive adhesive layer of the pressure-sensitive adhesive, double-coated tape, Japanese Patent Applications Laid-Open No. H07-187435 and No. H08-225178, e.g., specify that the thickness of the pressure-sensitive adhesive layer is 5 μm or more to 100 μm or less. Usually, the beltlike substrate and the meandering-preventive member are required to have non-tackiness and slipperiness in view of their use in image formation and the required mechanical properties. As materials that satisfy these requirements, materials having scanty adhesion as exemplified by fluorine-atom-containing resins are used in many cases, and it is actually difficult to achieve both image characteristics and bond strength if the thickness of the pressure-sensitive adhesive layer is in the range disclosed in the above publications. Thus, as an electrophotographic endless belt, its performance has not been satisfactory.
As discussed above, in regard to the attachment of the meandering-preventive member to the beltlike substrate, any method that can satisfy all of durability, attachment precision and bond strength has not been made available.
Flexibility of the meandering-preventive member is also an important characteristic when it is used as an attachment to the beltlike substrate.
Where the meandering-preventive member has a sufficiently high flexibility, it has a higher follow-up performance to the beltlike substrate, and hence the electrophotographic endless belt can smoothly rotate and travel. On the other hand, there is a problem that, when the electrophotographic endless belt has meandered to apply a twist force to the meandering-preventive member, the meandering-preventive member has no resilience strong enough to respond to its stress sufficiently and hence its meandering-preventing function may be damaged. Any too soft meandering-preventive member also has problems that it makes the meandering-preventive member itself have a low working precision and that it lowers attachment precision to the beltlike substrate.
On the contrary, where the meandering-preventive member has a poor flexibility, the effect of preventing the belt from meandering is sufficiently obtainable, but there is a problem that, because of a difference in stiffness (nerve or rigidity) of the meandering-preventive member from that of the beltlike substrate, the belt is hindered from traveling smoothly, and can not travel in a stable manner.
As a method which can solve the above problems, a method is available in which the electrophotographic endless belt is stretched over the rollers at a higher tension (belt tension). There, however, is a possibility that a higher belt tension causes creep of the electrophotographic endless belt to shorten its lifetime. There is also a possibility that too high a belt tension makes the electrophotographic endless belt meander more.
In particular, an electrophotographic endless belt set in a process cartridge may often undergo much vibration or be placed in an environment of high temperature and high humidity at the stage of distribution in the market, which environment is different from the environment in which it is used in the main body of an electrophotographic apparatus. In the case when it has been placed in such a severe environment for a long time, the pressure-sensitive adhesive may slip more, so that, when the process cartridge is set and used in the main body of an electrophotographic apparatus, the attachment precision of the meandering-preventive member may have already been out of order, so that the meandering-preventive member may often no longer function as such. For this reason, the above problem has more remarkably arisen in electrophotographic endless belts set in process cartridges.